This invention relates to current-limited power supplies.
In many applications it is desirable to convert a primary direct voltage to another direct voltage in an efficient manner. This problem often arises in the context of electrical equipment powered by the primary direct voltage system of a vehicle. The primary direct voltage system of the vehicle is often a battery having a relatively low direct voltage, whereas the equipment to be powered may require a higher voltage or a lower voltage The abbreviation dc (direct current) is often used to denote a direct voltage. Where the equipment to be powered requires a lower voltage at high current, a dc-to-dc converter may be used to avoid unwanted power dissipation in a voltage dropping resistor. Where the desired output voltage is higher than the primary voltage, the dc-to-dc converter is also desirable.
One way to implement a dc-to-dc converter is to use the primary direct voltage to energize an oscillator to produce an alternating current (ac), and to drive the primary winding of a transformer with the ac. The secondary voltage of the transformer at the appropriate voltage level is rectified and filtered to produce the desired direct voltage. It has been found to be more efficient to apply the primary direct voltage to a switched inductor, either with or without a transformer. In order to minimize the physical sizes of the inductor and of the transformer (if used), the switching often occurs at frequencies much higher than the 50 or 60 Hertz (Hz) power line frequency For example, dc-to-dc converters are often used in television receivers to produce the kinescope ultor direct voltage, and are switched in synchronism with the 15,750 Hz television horizontal deflection. In other applications, converters operating at frequencies in the hundreds of kilohertz or higher are sometimes used.
It is often convenient to incorporate feedback regulation into a switching dc-to-dc converter to compensate for load and other variations. This may be accomplished by sensing the direct output voltage of the converter, comparing it with a reference voltage to produce an error voltage, and controlling a pulse width modulated (PWM) driver by means of the error voltage. The pulse width modulated driver, in turn, establishes the duty cycle or the ratio of the ON (conductive) time period to the period of the converter switch, which in turn establishes the amount of energy stored in the inductor during each cycle for transfer to the output circuit, and therefore establishes the output voltage.
Constant-frequency, current-programmed dc-to-dc converters are described in an article entitled "Modelling and Analysis of Switching DC-to-DC Converters in Constant-Frequency Current-Programmed Mode" by Hsu et al., published 1979 by IEEE, pages 284-301. In current-programmed converters, the switched reactance is made conductive by a clock signal, and a ramp current increases until the switched reactance is rendered nonconductive by a comparator which compares the ramp current with an error voltage derived from a comparison of the converter output voltage with a reference voltage. These converters have simpler transfer functions than voltage-programmed converters, and therefore are easier to filter for stability and reduced ripple.
In switched dc-to-dc converters, relatively high instantaneous currents may flow during portions of each operating cycle. Ordinarily, the magnitudes of these currents are well within the operating limits of the elements themselves. However, in the event of a fault such as excessive load (too low a load resistance) or a short-circuited load, the current in particular components of the dc-to-dc converter may exceed allowable limits, thereby resulting in destruction of at least portions of the converter. It is customary to protect against excessive load currents by providing current sensing circuits arranged in a feedback loop. The current sensing circuits respond to current above a predetermined level in order to turn off or shut down the dc-to-dc converter, either temporarily or permanently.
Ordinarily, the dc-to-dc converter includes one or more semiconductor rectifiers connected for receiving a pulsating or alternating current from the switched inductor, possibly by way of a transformer. Because of the high operating frequency required for minimizing the physical size of the inductors and the transformer (if used), charge storage in the rectifiers gives the rectifiers the ability to conduct in either the forward or reverse direction for a short period after a period of conduction. In some circuits, the charge storage in such rectifiers may produce a momentary current pulse during each operating cycle. The current sense circuit may respond to the current pulse as though it were caused by an overcurrent in the load, such as that attributable to a short-circuit, thereby resulting in shut-down of the converter.
In order to prevent dc-to-dc converter shut-down as a result of current spikes occurring during ordinary cyclical operation, it is common to connect a low-pass filter to the current sensor to average the spike on the current sense signal, as is described in U.S. Pat. No. 4,524,412 issued June 18, 1985 to Eng. A low-pass filter used in this fashion introduces a delay into the feedback loop which, especially in the case of current-programmed dc-to-dc converters, may adversely affect normal operation as well as shut-down operation. The effect of such filter capacitors on the current sense operation is ameliorated in the arrangement of U.S. Pat. No. 4,453,193 issued June 5, 1984 to Huang et al. by applying a synchronously-variable voltage to the reference input of the comparators. This prevents shut-down of the converter due to so much of the normal operation current spike as manages to pass through the low-pass filter. However, the Hunan et al. arrangement does not reduce the delay attributable to the use of filters in the current sense circuit path. A current limited power supply is desired which provides effective current limiting in spite of cyclical current spikes, and which does not require low pass filters which have a significant time delay in the current sense circuit path.